1. Field of the Invention
The present invention relates to thermally-induced information-bearing-signal recording which usually is optically sensed; the invention preferably employs lasers and the like for providing the thermal energy to effect the recording.
2. Discussion of the Prior Art
The use of lasers for emitting heat-inducing coherent radiation and E-beams, which are heat-inducing beams commonly used with semiconductors, has been widely practiced in micromachining and signal recording applications. In the micromachining area, harmonic distortion that may be caused by the recording pattern of the thermally-induced recording is of no significance since information-bearing signals are not recorded. On the other hand, when lasers and the like are employed for recording information-bearing signals, the sensing of the recording is sensitive to the quality of the recording; that is, it is desired to maintain as high as possible signal-to-carrier and signal-to-noise ratios. The higher these ratios, the greater the probability of faithfully recording and reproducing information-bearing signals. The problems associated with such recording become acute as the areal densities increase which means that the energy levels in recording and sensing of information-bearing signals are reduced. As a result of the reduced energy levels involved in the recording and read-back, the signal-to-noise ratios are also reduced. Part of the noise is synchronous noise; that is, noise that is induced by either the read-back or recording processes. At relatively modest areal densities, the induced synchronous noise may be tolerated, at higher areal densities such noise is more troublesome. Accordingly, it is an object of the invention to reduce the synchronous noise in thermally-induced recordings.
In the micromachining area, U.S. Pat. No. 4,475,028, McGuire, Jr. et al., shows a multimode constant-potential pulsed welding apparatus having a hot-start feature in which the number of pulses per second is doubled at the beginning of the welding operation. The purpose of the "hot-start" is to facilitate initiating an arc at a lower current level than that provided by a lower-frequency pulsing, i.e., the hot-start, or increased energy, at the beginning of the welding period, is for reliably establishing an arc for welding.
There also has been substantial work in the laser machining of thin films and integrated circuits, as reported in the BELL SYSTEM TECHNICAL JOURNAL; for example, by the article by M. I. Cohen, et al., "Laser Machining of Thin Films and Integrated Circuits", BELL SYSTEM TECHNICAL JOURNAL, March 1978, pp. 385-405. This article teaches the use of solid-state lasers in fabricating tools which provide sharp definition and localized nature of working regions for allowing heating, melting or vaporizing minute amounts of material with minimal effect on adjacent material or components. The article teaches that it is the optical power density in the focused spot at the piece being machined rather than the laser-power output itself that determines the suitability of a laser for removing or ablating material. Further, the size of the affected zone on the target material being machined depends upon the thermal properties of the material as well as the laser beam's spot size and the energy-intensity distribution across the laser-emitted beam. Edge definition of the affected zone depends primarily on thermal properties of the target and the duration of the exposure. Reflectance of the surface of the material being machined may decrease abruptly as the material melts or reacts with the atmosphere, with subsequent laser-beam absorption occurring at greater efficiencies. Initial laser-output energy has to be sufficient to initially break down the surface. The machining taught by this article consists of a sequence of pulses of equal duration, size and shape and of substantially constant amplitude.
In another article, also in the BELL SYSTEM TECHNICAL JOURNAL, by D. Maydan, entitled "Micromachining and Image Recording on Thin Films by Laser Beams", August 1971, pp. 1761-1789, on page 1772, the relationship of the spot-size diameter to the size of the machining area is illustrated, see FIG. 6. An example of micromachining light pulses is shown in FIG. 7 on page 1773 as being light pulses of identical amplitude and of short equal durations. FIG. 8, on page 1774, shows that lines of differing widths may be provided using different laser-intensity outputs. FIG. 9, on page 1775, shows photographs of individual machining spots obtained from the pulsed-laser machining operation. The various pulse shapes used in the machining operations are shown in FIG. 13, page 1780. The use of laser machining using pulses of constant amplitude for video signal recording is shown in FIG. 17 on page 1784.
An article by Cohen, et al., entitled "Application of Lasers to microelectronic Fabrication", published by the New York Academic Press, 1968, in pp. 139-186, is an additional reference showing the effects of a laser beam on material being treated. At page 156, the article teaches that spot size increases with power- or light-intensity levels. Statements on page 164 compare laser welding with fusion welding. Page 167 discusses the effects of pulse duration. This article teaches that, in welding using a pulsed laser, one of the most important parameters to be considered is pulse duration. FIG. 17 on page 168 shows the effect of too long a duration and the resultant effect of the machining operation on the material. The authors state that it is possible to make satisfactory welds over a wide range of pulse durations so long as a minimum time required for adequate heating conduction is provided. While such a parameter is satisfactory for welding, it is certainly not an appropriate approach for high-areal-density information-bearing-signal recording. The authors also discuss the importance of maintaining proper energy control. On page 171, it is stated that the authors have previously suggested that a pulse shape that quickly rises to a peak value and then drops or decays to a lower level may be desirable for welding. This observation apparently is to prevent unintended splatter of the material as may be caused by overheating the area being subjected to a laser beam.
U.S. Pat. No. 3,962,558 to Kocher, et al., shows a pulsed-laser drilling machine having an initial high-energy pulse followed by a sequence of lower-energy pulses. Again, this arrangement is apparently selected to prevent undue splattering of the material being machined. An improvement over the Kocher, et al., patent is shown in U.S. Pat. No. 4,114,018 to Von Allmen, et al., which cites the machining techniques shown in U.S. Pat. No. 3,962,558 in FIG. 2 as well as the amplitude-decaying technique of Cohen, et al., supra, in FIG. 3. Von Allmen, et al., teach, as shown in FIG. 4 of the patent, that an optimum-current amplitude which is substantially constant is the preferable way of laser machining. This stated approach is confirmed by U.S. Pat. No. 4,410,968 to Frohbach, et al., which teaches that, for ablative information-bearing signal recording, the energy level of the recording laser beam should be such as to move the material using constant recording power such as to deform the film of the record-bearing medium for causing local redistribution of the material without vaporizing or splattering the material.
Another form of pulsed-laser recording is shown in U.S. Pat. No. 4,473,829 to Schouhamer Immink, et al., which uses overlapped circular-pulse-shaped beams for producing thermally-induced recordings of diverse lengths on a record-bearing medium. While rapidly pulsing a laser, or similar beam-emitting device, may be appropriate at relatively modest linear recording densities, at higher linear recording densities with a relatively rapidly-moving record-bearing medium, such pulsing can be difficult to achieve, i.e., it is more desirable to turn the laser or other beam-emitting source on and leave it on for the pulse duration. For variable-pulse-length recording operations, by not pulsing the laser, higher linear densities should be achievable. At high density, such overlapped recording pulses also create synchronous noise in the recording.
Pulsed lasers, in addition to ablative or material-redistribution recordings, have also been used for crystalline-to-amorphous phase-change recording. It is not readily apparent that the laser controls for ablative recording would apply to other forms of optical recording. For example, in the article in the IEEE TRANSACTIONS ON ELECTRON DEVICES by Ovshinsky, et al., entitled "Amorphous Semiconductors for Switching, Memory, and Imaging Applications", on pp. 91-105 of Vol. ED-20, No. 2, February 1973, page 97 in FIG. 10 shows electrical impedance characteristics of the amorphous-crystalline switchable material. In particular, there is shown an initial high electrical impedance value once the material has switched phase states; then the electrical impedance level drops as evidenced by the lowered voltage across the material. This article also shows the reversibility of optical effects of amorphous semiconductors that switch between amorphous and crystalline states. The term "semiconductors" is also to be applied to semimetals, which are more commonly used in the phase-change optical recording. Apparently constant current pulses were used in the recording rather than constant power pulses.
Another type of optical recording is the so-called magneto-optic disk such as described by Tsujiyama in ELECTRONICS AND COMMUNICATIONS IN JAPAN, Vol. 60-C, No. 7, 1977, in an article entitled "Magneto-Optical Disk Memory Utilizing Multilenses", on pp. 89-97. This article teaches recording on a magneto-optic record-bearing medium using pulsed lasers having a constant amplitude and shape for generating recording in circular recording areas. In a magneto-optic recording system, an ancillary magnetic field steers the remnant magnetization in one direction or the other while the laser is heating the material above the Curie point. Of interest here is that increases in energy density increase the size of the spot, hence the width and size of the recording. A series of pulses is used for recording short or long pulses, as shown in FIGS. 12 and 13 of this article.
The use of E-beams in semiconductor manufacture is quite well known. For example, see the IBM TECHNICAL DISCLOSURE BULLETIN article, "E-Beam Kinetic Focus for High-Speed Pattern Generation" by Koste, et al., December 1978, Vol. 21, No. 7, pp. 2768-2769. The use of E-beams for thermally recording identification indicia on a semiconductor chip is shown by P. M. Ryan in an article entitled "Automatic Serialization of Chips for Identification and Traceability", IBM TECHNICAL DISCLOSURE BULLETIN, Vol. 22, No. 1, June 1979, pp. 108-111.
European patent No. 45,117 shows examining the light reflected from a video disk for second harmonic content of the carrier frequency used to record the video signal. Control means are disclosed which adjust the recording laser beam power to minimize the second harmonic distortion of the carrier. This system appears applicable to video (analog) recording using a carrier frequency but does not show how to reduce synchronous noise in digital data base-band recording. The shape of any recording signal is not shown in this reference and it is not currently known to applicant.
The Frankfort et al. U.S. Pat. No. 4,562,567 also shows the examination of the light reflected from the optical record member, as in the European patent No. 45,117. FIG. 3C of this latter patent shows a recording beam intensity variation having a high-initial intensity and a reduced-final intensity. This patent teaching requires that the electronic circuits detecting the reflected light and for controlling the laser used to emit a recording beam to be faster than the time period of the shortest data bit to be recorded. This restriction limits the lineal data density of the recording. For example, pulse durations of about ten nanoseconds would not be capable using this latter system. That is, the propagation time of electrical signals through the detector and laser controller circuit plus the response time of solid-state lasers is high. To decrease the response time, the bandwidth of the detector and laser controller could be expanded which results in undesired noise being added to the signal being recorded. It is desired to provide a recording system that can handle recording in the picosecond range.
The above-cited last two references both require a change in reflected light intensity to operate. In recording in most magneto-optic recording media, there is no change in the reflected light intensity; such as found in the known ablative or phase-change recording media. Therefore, it is also desired to find a control system for reducing synchronous noise for magneto-optic media. (Recording in magneto-optic media merely raises the temperature of the recording layer to above the Curie point--destroy the magnetic properties of the media while above the Curie point to enable recording--rather than melting the active layer as in ablative and phase-change optical media.)